Globally projecting circuits, intrinsic to the SC/OT, that might support the generation of a relative priority map in the SC/OT ([27
]) have been sought, but not found [55
]. However, as recognized first by Sereno and Ulinski when studying the turtle midbrain [56
], the unusual anatomy of the isthmic nuclei is perfectly suited to serve this function. The isthmic nuclei, located beneath the SC/OT in the lateral tegmentum, contain two major circuits. One circuit consists of a population of cholinergic neurons that are connected reciprocally and topographically with the SC/OT. In birds, these neurons are in two adjacent nuclei called the nucleus isthmi pars parvocellularis (Ipc) and the nucleus semilunaris (SLu) (, blue [57
]); in mammals, these neurons cluster in a single nucleus called the parabigeminal nucleus [58
]. A second circuit, most thoroughly described in birds, consists of a population of GABAergic neurons that receive topographic input from the SC/OT and send projections back broadly to the space map in the SC/OT and to the cholinergic nuclei (, red [60
]). In birds, these neurons are in the nucleus isthmi pars magnocellularis (Imc); in mammals, the analogous neurons are thought to be in the lateral tegmental nucleus [59
Figure 4 The midbrain network for stimulus selection in birds. Schematic of cellular connections between the OT and the isthmic nuclei. OT neurons (green) send axons to cholinergic neurons (blue) in the Ipc and SLu and to GABAegic neurons (red) in the Imc. The (more ...)
The cholinergic isthmic circuit is thought to provide space-specific, feedback amplification of SC/OT responses, although this function has yet to be demonstrated directly. In owls, Ipc neurons are multimodal, sharply tuned for space, and respond according to the physical salience of stimuli [63
]. In cats, the responses of PBN neurons are enhanced by motor plans for orienting eye movements [64
]. Importantly, the responses of many Ipc neurons in owls are suppressed in a switch-like manner with increasing strength of a distant competing stimulus [65
], suggesting that Ipc activity contributes, at least in part, to switch-like responses in the OT.
The GABAegic isthmic circuit is thought to mediate global competition in the SC/OT, although this function has yet to be demonstrated directly. In pigeons, Imc neurons are responsible for long-range inhibition in the Ipc [66
]. Thus, the Imc is likely to mediate global competition in the OT both by regulating feedback amplification provided by the cholinergic isthmic circuit and by directly inhibiting OT neurons.
It has been proposed by many authors that the SC/OT, together with the isthmic circuits, form a module for the implementation of a winner-take-all evaluation of the most salient stimulus [56
]. A detailed model of this module accounted for some, but not all, of the key aspects of switch-like responses in the OT [67
], leaving open the question of the computational rules that implement improved peak discrimination in the midbrain network.